Electrophotographic systems (that is, systems in which a toner is deposited on a charged surface and subsequently transferred to a receiving sheet) employing liquid toners are well known in the imaging art, see for example Schmidt, S. P.; Larson, J. R.; Bhattacharya, R. in Handbook of Imaging Materials, Diamond, A. S., Ed.: Marcel Dekker, New York, 1991, pp 227-252 or Lehmbeck, D. R. in Neblette's Handbook of Photography and Reorography, Sturge, J., Ed.: Van Nostrand Reinhold, New York, 1977, Chapter 13, pp 331-387.
In most instances, the preferred solvent has been a high boiling hydrocarbon (for example, Isopar.TM. solvents, boiling range: 130-160.degree. C.) that has both a low dielectric constant and a high vapor pressure necessary for rapid evaporation of solvent following deposition of the toner onto a photoconductor drum, transfer belt, and/or receptor sheet. Rapid evaporation is particularly important for cases in which multiple colors are sequentially deposited and/or transferred to form a single image.
There are significant drawbacks to the use of hydrocarbon solvents with respect to adequate evaporation rates for high speed imaging applications, regarding low flash points (hydrocarbon solvents with boiling points less than 120.degree. C. typically have flash points below 40.degree. C.), environmental pollution, and toxicity. Similarly, chlorine containing solvents are undesirable from the standpoint of atmospheric pollution. It would be advantageous to employ a class of solvents with a higher evaporation rate than that of ordinary hydrocarbon solvents, lessened pollution concerns, non-flammability, and lower toxicity.
One class of solvents that can solve some of these problems consists of the perfluorinated (or highly fluorinated) solvents such as the Fluorinert.TM. solvents (3M Company), hexafluorobenzene and so on. While these solvents have many desirable physical properties that make them suitable as candidates in electrophotographic applications employing liquid toner dispersions, they are well known for their inability to dissolve or disperse most materials. Thus, in order to develop an electrophotographic process employing fluorinated solvents it is necessary to develop stable dispersions of pigment, polymer, and charging agents. This would have to be accomplished by preparation of organosol polymers that are capable of dispersing pigment in those solvents or to prepare latex emulsions of polymers that can disperse pigments, or by adsorbing highly fluorinated polymers onto pigments in fluorocarbon solvents.
Chlorofluorocarbons (e.g., Freon.TM.113) have been employed in solvents for electrophotographic liquid toner dispersions as described in Soviet Pat. No. 1,305,623.
Electrophotographic toners having perfluoroethylene as solvent have been described, but not actually used, in Japanese Kokai Nos. 59-114,549 and 59-114,550. However, perfluoroethylene is a gas at room temperature and wholly unsuitable as a solvent for electrophotography.
U.S. Pat. No. 5,026,621 discloses a toner for electrophotography comprising a color component and a fluoroalkyl acrylate block copolymer.
Liquid toners based on highly fluorinated solvents according to the present invention produce very quickly drying images (&lt;3 seconds) on the dielectric medium, so that successive imaging 3 and 4 colors can be performed at a rate of up to 3 pages of 4-color copy per minute on plain paper. The currently used developmental toners produced images that do not dry at a rate fast enough to produce the hard copy output at the required rate.
A general discussion of color electrophotography is presented in "Electrophotography," by R. M. Schaffert, Focal Press, London & New York, 1975, pp 178-190.